This invention generally relates to the coextrusion of films, and more particularly to a method, product and apparatus in which a tear path is formed during the coextrusion of a plurality of extrudates that make up the coextruded film, said tear path formation being accomplished by interrupting the extrusion of at least one of the plurality of extrudates before same is combined with the other extrudates into the coextruded film.
It is well known to modify thermoplastic films in a variety of manners so as to provide a means for opening packages made therewith without having to use a tool to sever the package, such typically being lines of severance taking the form of perforations, strips of one type of material molded into a bag of a differnt material, and score groove lines such as those described in U.S. Pat. No. 3,186,628. Some such structures are not suitable for forming impermeable packages in that they eliminate the integrity of the film by forming perforations therethrough or they can risk loss of such integrity by introducing scored lines having a depth that is not always uniformly controlled to the extent that the thickness of the film at some locations along the scored tear path can be excessively reduced, resulting in film breakthrough thereat during normal handling. Permeability, whether it be intentional as is the case for perforations through the film, or unintentional, such as that sometimes brought about by an excessively deep location along a scored line, are not suitable for applications where it is critical to maintain the integrity and/or impermeability of the film. This is especially important for applications where the film is to be used in preparing gas-impermeable enclosures or containers demanding relatively large lateral strength values. The difficulty in controlling the depth of score groove lines also causes grooving to a depth less than that intended for a particular end use, leading to unsatisfactory products or to scrapping by the manufacturer.
There is accordingly a need for films and enclosures having a tear path that is imperforate and that has a uniformity of depth and width that provides packaging materials and products having a tear path depth, configuration and tear resistance or strength that can be varied as desired in a reliable and exceptionally uniform manner. It would also be produced on equipment and by a method that does not require the expenditure of substantial capital over that readily available to a manufacturer of coextruded film products.
Such desirable attributes and properties are achieved in accordance with the present invention by providing films that are coextruded from at least two extrudates, at least one of which has its extrusion flow interrupted at one or more narrow locations so as to form an elongated void within the extrudate, such elongated void, which is introduced before the extrudates are laminated or combined into a coextrudate, forming a uniform tear path in the solidified coextrudate having a depth, width and length as desired for the particular intended end use. Each desired extrudate flow interruption is typically provided by a tine interposed within the path of molten extrudate in close proximity with, directly adjacent to and/or inside of the extrusion orifice through which the extrudate flows.
It is accordingly a general object of this invention to provide a film having a tear path therewithin.
Another object of this invention is to provide an improved film, packaging material, and enclosure product that is coextruded with a tear path of uniform depth.
Another object of the present invention is an improved apparatus and method incorporating a closely controlled manner of tear path formation.
Another object of the present invention is an improved apparatus, method and product incorporating coextrusion techniques that are modified so as to provide a tear path within the solidified coextrudate.
Another object of this invention is an improved apparatus, method and product incorporating a tear path that is variable as desired to provide a preselected, substantially uniform and reproducible tear path strength.